Biodiesel obtained from alternate renewable resources has received considerable attention for dealing with the deteriorating situation of world energy supply. Biosynthesis of lipids by microorganisms such as bacteria, yeasts and algae is well established. There are many commercial applications of lipids in the area of food industry, pharmaceuticals, and cosmetics. Lipids such as triglycerides, serve as precursors for biodiesel. Triglycerides, once extracted, can be converted into biodiesel through trans-esterification reactions. Many species of oleaginous microorganisms have been reported to accumulate significant amounts of lipids (Meng et al., Renewable Energy, 2009, 34, 1-5).
Microalgae are a highly specialized group of oleaginous microorganisms adapted to various ecological habitats. Under stress and adverse environmental conditions, many species of microalgae alter their lipid biosynthetic pathways favoring the formation and accumulation of triglycerides (20-50% dry cell weight). After being synthesized, triglycerides are deposited in to densely packed cytosolic lipid bodies. (Hu et al., The Plant Journal, 2008, 54, 621-639). Triglycerides generally serve as energy storage molecules in microalgae.
Significant amount of research has been carried out to identify and develop efficient lipid induction techniques in microalgae by modulating various environmental factors such as nutrient availability, osmotic pressure, radiation exposure, pH, temperature, exposure to heavy metals and other chemicals (Sharma el al., Energies, 2012, 5, 1532-1553).
WO1989000606 describes the production of omega-3 (n−3) lipids in microalgae cells subjected to limiting quantities of nitrogen and phosphorous in the medium.
WO2001054510 describes enhanced production of microbial lipids in eukaryotic microorganisms grown in a non-alcohol carbon source and a limiting nutrient source.
U.S. Pat. No. 8,475,543 describes a process of production of bio-fuels from algae including cultivating oil-producing algae by promoting sequential photoautotrophic and heterotrophic growth. The heterotrophic growth stage is initiated using a stress induction mechanism including at least one of light deprivation, nutrient deprivation, injection of a reactive oxygen source, a lipid trigger, and chemical additives. Further, the process describes extracting the oil from the oil-producing algae by lysis of the cells.
Production of biodiesel from microalgae is an emerging field and appears to be a potential alternative bio-resource. However, current technology involves intra-cellular production of triglycerides or lipids in the microalgae, thus creating a roadblock in downstream processing i.e., the microalgae cells have to be harvested, dried and ruptured either by mechanical or biochemical methods for the extraction of lipids, thereby increasing time and cost factors.
In light of current available technology and scientific advances, there exists a need for developing a cost-effective and efficient process that eliminates the need for harvesting and rupturing the microbial cells for extraction of the intra-cellular lipids.